With the increasing demand for the higher printing speed in the plain paper copier (PPC) and the laser printer, the needs for the multi-beam semiconductor laser device have been rapidly increasing. Since the multi-beam semiconductor laser device has light-emitting portions arranged single-dimensionally or two-dimensionally, the number of scanning beams can be increased, and thus it has the advantage that the high-speed printing can be achieved.
The general structure of the multi-beam semiconductor laser device will be described with reference to FIG. 1. FIG. 1 shows a four-beam semiconductor laser device as an example of the multi-beam semiconductor laser device. As shown in FIG. 1, a semiconductor laser element array (laser chip) 8 including a semiconductor substrate 11 made of GaAs or the like (hereinafter, simply referred to as substrate), a semiconductor layer 2 having light-emitting portions 7 arranged in an array, an n type electrode 1 and p type electrodes 3 separated into stripes is bonded with solders 4 on submount electrodes 5 formed into stripes on a submount 6. A metal layer 10 for heat dissipation made of Au plating or the like is formed on each surface of the p type electrode 3.
The submount 6 is bonded to a heat sink (not shown) made of Cu via solder or others. The submount 6 has a function to relax the thermal stress caused by the difference in a linear expansion coefficient between the heat sink and the semiconductor laser element array 8 and improve the heat dissipation. Therefore, a material having high thermal conductivity and a thermal expansion coefficient close to that of the substrate 11, for example, SiC, Si, CuW or AlN is used as a material of the submount 6.
The above-mentioned mounting method in which the light-emitting portions 7 of the substrate 11 are directed to the submount 6 side is called a junction-down mounting, and this method has an advantage of efficiently dissipating the heat generated in the light-emitting portions 7 to the submount 6. However, it has been known that since stress is likely to be applied to the bonding portion between the substrate 11 and the submount 6 in this junction-down mounting, strain is applied to the light-emitting portions 7 due to the thermal stress at the time of mounting, which causes the variations in optical properties. In particular, since it is required to realize the laser elements having uniform optical properties by reducing the relative difference in properties of each beam such as wavelength, polarization angle, light-emission efficiency and optical output in the case of the multi-beam semiconductor laser device, it is an important issue to reduce the relative difference in strain applied to the light-emitting portions by reducing the thermal stress at the time of mounting.
However, when the multi-beam semiconductor laser device in which light-emitting portions are arranged single-dimensionally is fabricated, the polarization directions of the beams are varied and the relative difference in polarization angle (rotation of the angle of polarization plane with respect to a plane parallel to an active layer in a semiconductor layer) occurs. Further, it is known that when the shear strain is applied to the semiconductor layer, the polarization direction of the beam is rotated in proportion to this shear strain (M. A. Fritz, IEEE Trans. Comp. Package. Technol., 27 (2004) p. 147), and it is considered that the above-mentioned relative difference in polarization angle is caused by the relative difference in shear strain in each of the light-emitting portions.
The above-mentioned relative difference in shear strain will be described with reference to FIG. 1 to FIG. 3. Note that the case where a linear expansion coefficient of the substrate 11 is larger than that of the submount 6 will be described here.
The semiconductor laser element array (laser chip) 8 is bonded to the submount 6 with the solders 4 made of Au—Sn or the like under the temperature of 200° C. to 300° C. When the temperature falls to ambient temperature after the solder bonding (after mounting), since the submount 6 contracts less than the substrate 11 (has a smaller linear expansion coefficient), the semiconductor layer 2 is pulled outwardly in a horizontal direction on the p type electrode 3 side near the submount 6, but the semiconductor layer 2 contacts inwardly in a horizontal direction on the n type electrode 1 side (see FIG. 2). The horizontal direction mentioned here is defined as a direction parallel to the bonding surface between the semiconductor layer 2 and the substrate 11, and the vertical direction is defined as a direction vertical to the bonding surface between the semiconductor layer 2 and the substrate 11, in other words, a direction vertical to a resonator direction of the semiconductor laser element array 8.
Parallelograms shaded with hatching in FIG. 2 show the state of shear deformation of each of the light-emitting portions 7, and FIG. 3 shows the plotted shear strains of the light-emitting portions 7. Note that the light-emitting portions 7 are numbered as LD1, LD2, . . . in order from the left end in FIG. 2 and FIG. 3. As is evident from FIG. 3, the magnitude of the vertical and horizontal shear strain after the mounting differs in each of the light-emitting portions 7, and relative difference in shear strain occurs. Note that, when the linear expansion coefficient of the substrate 11 is smaller than that of the submount 6, plus and minus of the shear strain in the light-emitting portions 7 are reversed in FIG. 3. As described above, since different shear strains are applied to each of the light-emitting portions 7 after the mounting, the polarization angles of the beams differ and the relative difference in polarization angle (difference in polarization angle between beams) occurs in the multi-beam semiconductor laser device.
As a method of suppressing the rotation of the polarization angle and a method of reducing the strain applied to the light-emitting portions after the mounting described above, Japanese Patent Application Laid-Open Publication No. 2002-246696 (patent document 1) has suggested the means for addressing the displacement of a polarization direction of a laser light with respect to a direction parallel to an active layer of a laser chip. In this suggestion, a light-projecting point is located at a position intentionally displaced from a central position of a chip in a width direction, thereby addressing the displacement of the polarization direction.
Also, Japanese Patent Application Laid-Open Publication No. 07-202323 (patent document 2) has suggested the means for obtaining the multi-beam semiconductor laser device having uniform optical properties. In this suggestion, a pseudo laser element which does not emit light at the time of use is formed outside the region where a plurality of semiconductor laser elements are formed, and this pseudo laser element is made to absorb the thermal stress, thereby obtaining the multi-beam semiconductor laser device having uniform optical properties.